Aspects of the cell biology an immunology of host-parasite interactions among several intra- and extracellular parasitic protozoa are being investigated. Leishmania sp. and Trypanosoma sp. are being used as models of intracellular and extracellular parasitism, respectively. Emphasis is placed upon: 1) determining some of the basic chemical and antigenic properties of parasite surface membranes; 2) ascertaining the nature and extent of the interactions of parasite surfaces with specific host cell types; 3) defining the basic mechanisms involved in the intracellular survival and multiplication of parasites within host cells; and 4) attempts to determine the means by which parasites circumvent host immuno-defense systems. Techinques employed in these studies include: subcellular fractionation, ultracyto- and immunochemistry, electron microscopy, affinity chromatography, acrylamide gel gradient electrophoresis, lectin assays, radio isotope labeling, gel immunoassays, and in vitro cell culture.